Development of secondary batteries used as a driving power source has been proceeding in recent years as they are considered as one of important key devices. Among them, lightweight and small-sized nickel-hydrogen storage batteries and lithium ion secondary batteries having high energy density have widely been used as a driving power source of consumer devices such as cellular phones, and electric vehicles and electric tools. Recently, the lithium ion secondary batteries have been considered as a possible driving power source and active development has been conducted for higher capacity and high output power.
The secondary batteries used as the driving power source require a high output current. From this point of view, a secondary battery with a devised structure, particularly with a devised current collecting structure, has been proposed.
A current collecting structure formed by, for example, winding a positive electrode plate and a negative electrode plate with a separator interposed therebetween and welding the positive and negative electrode plates to current collector plates through current collector tabs, has been employed for the purpose of increasing an electrode area. In this current collecting structure, however, the current collector tabs have a high electrical resistance, and therefore current collecting efficiency is impaired. It has been difficult to apply this structure to the driving power source that requires a high output current.
A “tabless” current collecting structure formed by joining whole end portions of the positive and negative electrode plates to the current collector plates is suitable for high current discharge because it can reduce the electrical resistance. However, for the tabless current collection, it is necessary to join the end portions of the positive and negative electrode plates to the current collector plates with reliability.
FIGS. 16(a) and 16(b) show a current collecting structure described in Patent Document 1. FIG. 16(a) is a sectional view of a current collector plate 40 and FIG. 16(b) is a sectional view of the current collector plate 40 with the end portions of the positive electrode plates (or the negative electrode plates) 41 joined thereto.
As shown in FIG. 16(a), grooves 40a are formed in the surface of the current collector plate 40 at positions corresponding to the end portions of the positive electrode plates (or the negative electrode plates) 41. The end portions of the positive electrode plates (or the negative electrode plates) 41 are inserted into the grooves 40a. The end portions of the positive electrode plates (or the negative electrode plates) 41 are joined to the current collector plate 40 by fusing parts of the current collector plate 40 forming the grooves 40a. 
In the current collecting structure formed by this method, the end portions of the positive electrode plates (or the negative electrode plates) 41 are welded to the current collector plate 40 at joints 42 as they are buried in metal comprising the current collector plate 40. Thus, the end portions of the positive electrode plates (or the negative electrode plates) 41 can reliably be joined to the current collector plate 40.
According to the aforementioned method, however, the grooves 40a in the current collector plate 40 have to be formed in correspondence with the arrangement of the positive electrode plates (or the negative electrode plates) 41 of the electrode group. This requires a technique for placing the end portions of the positive electrode plates (or the negative electrode plates) 41 in alignment with the grooves 40a to be inserted therein. As a result, production processes may be complicated and production cost may increase.
Patent Document 2 discloses a simple method that allows joining of the end portion of the positive electrode plate (or the negative electrode plate) 41 to the current collector plate 40 without alignment.
FIG. 17 is a sectional view of a current collecting structure of a secondary battery disclosed by Patent Document 2. As shown in FIG. 17, a positive electrode plate 51 and a negative electrode plate 52 slightly shifted from each other in the vertical direction are wound with a separator 53 interposed therebetween. End portions 51a and 52a of the positive and negative electrode plates 51 and 52 protruding from the separator 53 are welded to current collector plates 60 and 61, respectively. The end portions 51a and 52a of the positive and negative electrode plates 51 and 52 are pressed in a winding axis direction (the vertical direction in the FIG. 17) so that they form flat portions, and the flat portions are welded to the current collector plates 60 and 61, respectively.
In the current collecting structure formed by this method, the flat portions formed by the positive and negative electrode plates 51 and 52 themselves are brought into contact with the current collector plates 60 and 61 and welded thereto. Therefore, the end portions of the positive and negative electrode plates 51 and 52 can be joined to the current collector plates 60 and 61 by a simple method without alignment.
The aforementioned method has the following drawbacks from the viewpoint of increasing the capacity and reducing the size of the secondary battery. Specifically, when a current collector foil constituting the positive or negative electrode plate 51 or 52 is thinned down, the thinned down foils decrease in mechanical strength. This makes it difficult to form the uniformly bent flat portions by pressing the positive or negative electrode plate 51 or 52. In particular, aluminum and copper are used as the current collector foil used in the positive and negative electrode plates 51 and 52 of the lithium ion secondary battery. Therefore, when the current collector foil is thinned down to about 20 μm or less, for example, it will be very difficult to form the flat portions by pressing. Further, when the end portions 51a and 52a (portions uncoated with a material mixture) of the positive and negative electrode plates 51 and 52 are warped due to the pressing, the material mixture applied to portions 51b, 52b of the current collector foil coated with the material mixture may come off or break.
Patent Document 3 discloses a technology that allows joining of the end portion of the positive or negative electrode plate to the current collector plate even when the current collector foil constituting the positive or negative electrode plate is thinned down.
FIG. 18 is a perspective view illustrating the structure of a current collector plate 70 disclosed by Patent Document 3. As shown in FIG. 18, a first projection 70a and a second projection 70b project toward the opposite directions from the surfaces of the flat current collector plate 70. With the end portion of the positive electrode plate (or the negative electrode plate) 80 kept in contact with the second projection 70b, energy is applied to the first projection 70a to fuse the first projection 70a, part of the body of the flat current collector plate 70 and the second projection 70b so that the end portion of the positive electrode plate (or the negative electrode plate) 80 is joined to the current collector plate 70.
Regarding the current collecting structure formed by this method, the end portion of the positive electrode plate (or the negative electrode plate) 80 is just brought into contact with the second projection 70b of the current collector plate 70 so that it is joined to the current collector plate 70 with a fused material obtained by fusing the current collector plate 70. Therefore, even when the current collector foil constituting the positive electrode plate (or the negative electrode plate) 80 is thinned down and decreased in mechanical strength, the end portion of the positive electrode plate (or the negative electrode plate) 80 can be joined to the current collector plate 70 without applying any load to the current collector foil.    [Patent Document 1] Published Japanese Patent Application No. 2006-172780    [Patent Document 2] Published Japanese Patent Application No. 2000-294222    [Patent Document 3] Published Japanese Patent Application No. 2004-172038